Analysis of photoinduced magnetization in a (Co, Fe) Prussian blue model

The electronic properties of a modified (Co, Fe) Prussian blue compound, which exhibits interesting photoinduced magnetic properties, are analyzed. For band structure calculations we set realistic models (for the ground and excited states) that involve interstitial potassium ions, crystal water, and lattice defects. The top of the "t_2g" block consists mainly of Fe 3d orbitals, the low-lying "e_g(1)" block Co 3d. and the high-lying "e_g(2)" block Fe 3d. Because of the influence of crystal water coordinating to the Co ion, the "e_g(1)"-block bands split into two parts; one is the "e_g(1)(N)" block originating from the Co-NC fragment and another the "e_g(1)(O)" block from the Co-OH₂ fragment. The energy of visible light causing the magnetization enhancement is close to the computed "t_2g(Fe-based)-e_g(1)(N)" and "t_2g(Fe-based)-e_g(1)(O)" separations in the ground-state model. It is reasonable from DOS (density of states) analyses that irradiation with visible light causes an electron transfer from the Fe(II) to the Co(III) ions, through which the magnetization is effectively enhanced. Such an electron transfer is not a d-d transition on the same metal ion so that the Laporte rule may not be applied, or if applied, this rule may be relaxed in the (Co, Fe) Prussian blue.